AuthorTopic: Effects of diffraction (Read 6984 times)

Yes, they are maximum values. Denser sampling will usually help because it provides a better oversampling of the diffraction pattern, which in turn allows more accurate restoration by deconvolution sharpening.

Yes, that's how it happens to work out for high contrast detail. But do keep in mind that lower contrast subject matter will be mostly lost already at wider apertures than these limiting ones. Stopping down will kill low contrast micro-detail long before it kills all micro-detail (of any level of contrast). For the D800 it starts at f/5.6 and for the D3 it starts at f/9 .

Cheers,Bart

Hi BartHere we're getting into even higher degrees of speculation. How low is low-contrast? Also, at some predefined level of lowness, the strength of the camera's AA filter will have a more significant effect on the results. Many people were surprised to see how little resolution difference there seemed to be between the D800 and the D800E in the sample images on display. At a particular low level of contrast, I imagine the D800E would produce a more obvious improvement over the D800, than it does at high contrast.

What I propose is a test chart with the usual B&W lines spaced at progressively smaller intervals, then another columm on the same sheet containing the same size and spacing of lines, but dark grey and white lines, then another column of medium grey and white lines, then pale gray and white lines, then very pale grey and white lines.

Such a chart would enable us to quantify more precisely the different capacities of different cameras, with different strengths of AA filters and different pixel densities, to handle predefined levels of low-contrast micro-detail at specific F stops.

However, I see another problem here. It is understood that one would try to use the same lens on both cameras when doing such a test, but the choice of lens may significantly affect the results, and I'm not referring just to the quality of the lens, in terms of good or bad, cheap or expensive, but rather the lens MTF characteristics and its design.

For example, certain high-contrast lenses, such as some of the Carl Zeiss lenses, are very appealing (and very expensive of course) because they have a high MTF response at relatively low frequencies, say up to 40 lp/mm. However at higher frequencies up to the Nyquist limit of the D800, say 100 cy/mm, the MTF response may be significantly lower in the Zeiss lens than another design of lens, whether the other lens is cheaper or not.

Therefore, when comparing the handling of low-contrast micro-detail with the D800 and D3, for example, the high-contrast lens would favour the D3 and the low-contrast lens would favour the D800. In other words, the relatively high MTF response of a low contrast lens at 70 to 100 lp/mm would be irrelevant with regard to the D3 because the D3 cannot resolve any more than 60 lp/mm, whatever the circumstances. But that additional contrast at high resolution could have a visible impact on the D800 results.

ErikThanks for the summary!I certainly can verify the accuracy of your conclusions with both the IQ180/Rodenstock HR's and the Nikon D800E/Leica R's The sweet spot is f5.6 to f8.0 but f4 to f16 is certainly usable and with proper sharpening quite goodMarc

Hi BartHere we're getting into even higher degrees of speculation. How low is low-contrast?

That's where MTF curves come to the rescue. When the MTF response for a given lens/aperture/camera combination is e.g. 10% for a certain spatial frequency, then a subject with 20% contrast and that spatial frequency of detail will be rendered at 2% resulting contrast. It's only just visible. If the subject itself only had some 5% contrast, then the resulting 0.5% would probably not even be resolved anymore.

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Also, at some predefined level of lowness, the strength of the camera's AA filter will have a more significant effect on the results. Many people were surprised to see how little resolution difference there seemed to be between the D800 and the D800E in the sample images on display. At a particular low level of contrast, I imagine the D800E would produce a more obvious improvement over the D800, than it does at high contrast.

Indeed, such low contrast micro-detail will benefit from any boost in the MTF. However, at the boundaries of resolution we are also faced with Demosaicing false color artifacts due to the aliased input signal. So it depends a bit on how things are implemented in the Rawconverter, e.g. does noise reduction differentiate between high and low contrast micro-detail?

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What I propose is a test chart with the usual B&W lines spaced at progressively smaller intervals, then another columm on the same sheet containing the same size and spacing of lines, but dark grey and white lines, then another column of medium grey and white lines, then pale gray and white lines, then very pale grey and white lines.

Such a chart would enable us to quantify more precisely the different capacities of different cameras, with different strengths of AA filters and different pixel densities, to handle predefined levels of low-contrast micro-detail at specific F stops.

That wouldn't be too difficult to make based on e.g. my resolution test chart, just lower the contrast by adjusting the output levels more towards mid-gray before assigning the output profile to it and printing it.

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However, I see another problem here. It is understood that one would try to use the same lens on both cameras when doing such a test, but the choice of lens may significantly affect the results, and I'm not referring just to the quality of the lens, in terms of good or bad, cheap or expensive, but rather the lens MTF characteristics and its design.

Correct, we can only draw absolute conclusions based on a given combination of equipment. There is also variation between lens copies. But there are of course general findings that describe a cross-section of the population. One can use the slanted edge features on the test chart for that purpose.

For those who don't have Imatest software at their disposal, one can also use my Slanted Edge Analysis tool. When the Blur radius is calculated, one can use section 4. of the tool page, and produce an MTF curve from that Gaussian blur.

That section 4 on the tool's page can also be used independently from the 3 prior steps, as a means to test various scenarios. What I have sofar found is that blur radii of 0.7 are quite common for modern lenses around their optimal aperture of f/5.6 or thereabout. Stopping down to f/11 - f/16 will produce blur radii in the order of 1.0 or a bit more, all before sharpening. Sharpening of course attempts to reduce the blur to very small values, a radius of 0.39 corresponds to an edge profile rise from 10-90% in approx. 1 pixel distance.

Hisorry to interrupt this high-end discussion with a very trivial problem. Erik, I can't read your web site - when I adjust the browser to show the text lines all the way to the right, the characters are way too small for reading. When I enlarge them, the lines are cut off at right, and it does not help to use the horizontal scroll bar.Kind regards - Hening.

Correct, we can only draw absolute conclusions based on a given combination of equipment. There is also variation between lens copies. But there are of course general findings that describe a cross-section of the population. One can use the slanted edge features on the test chart for that purpose.

It's a pity that no-one is producing MTF results for the lens only, at frequencies that range as far as the Nyquist limit of modern DSLRs, say 10, 20, 40, 80 & 120 lp/mm. For best low-contrast micro-detail, the lens with the highest MTF response at 80 and 120 lp/mm might be preferred.

I suspect that most lenses are designed to make the coarsest detail look best, ie. snappy and contrasty, at the sacrifice of finer and fainter detail which gets lost. One can always enhance coarse detail that lacks contrast, in post processing, but nothing can be done regards detail that was never captured as a result of both the contrast of the subject matter being too faint and the MTF response of the lens at the same spatial frequency being too low.

I wish Photodo had continued with their MTF testing of the lenses only, and added a few higher frequencies instead of discontinuing the whole process. I used to find their results useful.

The 10, 20, 40 standard of MTF presentation was developed in the film days. It's widely used both Zeiss and Leica present MTF in that formate.

Rodenstock and Schneider added curves for 60 lp/mm on some of their HR lenses for digital.

Olympus is using another set, taking their smaller format into account.

My guess is that fine detail is less important than high MTF at lower frequencies at normal viewing distances. A lot of research has gone into that. But now we often pixel peep both on screen and print,

A lens that "hits the diffraction limit" early is probably a good lens. This can be used as a hint. A lens that needs to be stopped down to f/11 will not produce very good fine detail.

I agree that it would be nice to have MTF curves available that go to Nyquist. As an alternative it would be possible to show LW/PH or cy/px data for low MTF (like 20%).

Vendors MTF data seems to be inconsistent. I presume that in many cases the MTF curve is more of an artists impression than measured lens data.

Regarding the old Photodo data, it was measured by Hasselblad. Unfortunately the Photodo name was sold and the Photodo now in existence seems to provide little data of interest.

We have a periodical here in Sweden that still measures MTF at Hasselblad (AFAIK), but they only publish data for 20 lp/mm (although they measure up to 60 lp/mm).

DxO mark seems to have decent lens tests, and so has SLRGEAR and Dpreview. And we have Photozone, of course.

It's a pity that no-one is producing MTF results for the lens only, at frequencies that range as far as the Nyquist limit of modern DSLRs, say 10, 20, 40, 80 & 120 lp/mm. For best low-contrast micro-detail, the lens with the highest MTF response at 80 and 120 lp/mm might be preferred.

I suspect that most lenses are designed to make the coarsest detail look best, ie. snappy and contrasty, at the sacrifice of finer and fainter detail which gets lost. One can always enhance coarse detail that lacks contrast, in post processing, but nothing can be done regards detail that was never captured as a result of both the contrast of the subject matter being too faint and the MTF response of the lens at the same spatial frequency being too low.

I wish Photodo had continued with their MTF testing of the lenses only, and added a few higher frequencies instead of discontinuing the whole process. I used to find their results useful.

A lens that "hits the diffraction limit" early is probably a good lens. This can be used as a hint. A lens that needs to be stopped down to f/11 will not produce very good fine detail.

Hi Erik,

Yes, that seems to be a fair assessment. To avoid confusion though, I'd rather call it "starts being affected by diffraction more than by residual lens aberrations" than limited. There is only one limit, and that's the absolute diffraction limit where resolution is lost even for high contrast signals.

A lens stopped down beyond it's optimal aperture (often one or two stops narrower than wide open) will start to lose overall contrast due to diffraction, and the highest spatial frequencies will suffer most because they already have the lowest contrast. However, as long as the input signal has a high enough contrast at those highest spatial frequencies, there will still be a resolved final signal, it is not limited. It's only limited when at any signal contrast the resulting signal is always lost, and that is wavelength and aperture dependent.

Because our Bayer CFA sensors and Raw converters favor Luminance over Chroma resolution, the above mentioned list of limits for 564 nm wavelengths allows a reasonable prediction of what to expect. As such it is also useful to predict 'total immunity' to aliasing artifacts. Without signal beyond the Nyquist spatial frequency, there can be no aliasing. This can be useful in situations (with stationary subjects) where it is easier to shoot an additional truely diffraction limited shot to be used in postprocessing of problematic subject matter, than to spend valuable time on trying to fix luminosity moiré.

Yes, that seems to be a fair assessment. To avoid confusion though, I'd rather call it "starts being affected by diffraction more than by residual lens aberrations" than limited. There is only one limit, and that's the absolute diffraction limit where resolution is lost even for high contrast signals.

A lens stopped down beyond it's optimal aperture (often one or two stops narrower than wide open) will start to lose overall contrast due to diffraction, and the highest spatial frequencies will suffer most because they already have the lowest contrast. However, as long as the input signal has a high enough contrast at those highest spatial frequencies, there will still be a resolved final signal, it is not limited. It's only limited when at any signal contrast the resulting signal is always lost, and that is wavelength and aperture dependent.

Because our Bayer CFA sensors and Raw converters favor Luminance over Chroma resolution, the above mentioned list of limits for 564 nm wavelengths allows a reasonable prediction of what to expect. As such it is also useful to predict 'total immunity' to aliasing artifacts. Without signal beyond the Nyquist spatial frequency, there can be no aliasing. This can be useful in situations (with stationary subjects) where it is easier to shoot an additional truely diffraction limited shot to be used in postprocessing of problematic subject matter, than to spend valuable time on trying to fix luminosity moiré.

That is not too difficult to answer, denser sampling will get the most out of any optical projection. However, physics sets a hard absolute diffraction limit, based on wavelength and aperture. No other parameters play a role.

For a 564 nm wavelength (wich is a nice Luminance weighted average between Red, Green, and Blue) and a round aperture, diffraction will limit spatial frequency resolution in the focal plane by a zero modulation at resolutions beyond:

f/32 is limited at 55.4078 cycles/mm, a sample pitch of 9.0 micron

f/28 is limited at 62.1932 cycles/mm, a sample pitch of 8.0 micron

f/25 is limited at 69.8095 cycles/mm, a sample pitch of 7.2 micron

f/22 is limited at 78.3585 cycles/mm, a sample pitch of 6.4 micron

f/20 is limited at 87.9544 cycles/mm, a sample pitch of 5.7 micron

f/18 is limited at 98.7255 cycles/mm, a sample pitch of 5.1 micron

f/16 is limited at 110.816 cycles/mm, a sample pitch of 4.5 micron

There is zero resolution possible beyond those spatial frequencies, none, nada. The indicated sample pitches have a Nyquist frequency that limits resolution at that same maximum spatial frequency, and as a consequence of diffraction there cannot be any aliasing because there is no signal beyond Nyquist. These are the absolute optical and sampling limits.

Because our sensors are not point samplers but area samplers, and our lenses are not perfect, the optical limits are even a fraction lower, but the above limits cannot be broken no matter how close the small sampling area gets to resembling a point sampler, or how good our lenses are. Only by using shorter wavelength light can we squeeze a bit more resolution out of our diffraction limiting optics. That's why chip manufacturers use UV and X-ray wavelengths to expose the photo resist masking layers, to beat the absolute limits of diffraction.

And again, near those diffraction limits there will only be resolution for very high contrast features because the MTF response is so low. Micro contrast is increasingly limited once the diffraction pattern diameter starts to exceed 1.5x the sensel pitch. A very rough rule of thumb tells us that, for wavelengths around 555 nm, that starting point of low micro contrast loss is at the sensel pitch in microns plus 10%, so for a D800 with a 4.88 micron sensel pitch that would be at approx. f/5.6 . Therefore, for the scenario of magnification capability based on cycles/mm, the per pixel contrast at maximum (for a given purpose) magnification will be low, unless the subject contrast was very high.

Bart,

Very useful data. I presume that 0 MTF represents the Dawes limit. To illustrate some of your points and invite further discussion I will present some of my own data for the Nikon D800e and the MicroNikkor 60mm f/2.8 AFS derived from your resolution chart and calculating the Gaussian radius with your method. I published the shots of your chart earlier, but have revised the resolution at f/5.6 as per your suggestion and also the other resolution figures. Since the Gaussian radius was excessive at f/2.8 and improved with stopping down, I suspect some degree of defocus may be present.

The table summarizes the results. MTF at the Rayleigh limit and at 50% contrast are from Roger Clarks' site. At f/5.6 resolution is close to the Nyquist limit for low contrast (approximately the Rayleigh limit), but resolution at 50% contrast is appreciably lower as measured by Imatest, but can be improved with sharpening. At f/16 and smaller apertures, resolution by your method and by Imatest are close to the theoretical limits.

Examination of the images from your resolution chart are interesting. The red circle indicates the Nyquist limit. At f/16 the system resolves close to the Nyquist limit and contrast could be improved by careful deconvolution sharpening. At f/32 there is complete loss of any detail near Nyquist and deconvolution sharpening would probably be of little benefit.

The edge response as measured by imatest demonstrates why an increased sharpening radius is needed for f/32. The sharpening algorithm needs to look out further to areas where there is a reasonable semblance of white and black. A narrow radius would encompass only gray values.